The present invention relates to an apparatus for measurement of the reception time of a pulse in a receiving system comprising at least one receiving channel with a nonlinear transmission response, which receiving channel supplies a received signal at its output.
The invention further relates to an apparatus for measurement of the reception time of a pulse with high dynamic range in a receiving system comprising at least two parallel receiving channels of differing sensitivity, which receiving channels produce at their outputs a set of time-parallel received signals.
Furthermore, the invention also relates to methods for measurement of the reception time of a pulse in systems of this kind.
The precise measurement of the reception time of a pulse is of enormous practical significance in surveying applications, for example for range finding through measurement of the transit time of an optical pulse. Even a time measurement error of 1 ns equates here to a distance measurement error of 30 cm, which is unacceptable for high-precision requirements. Further precision applications are found in, for example, satellite navigation, in which the reception time of satellite radio pulses has to be determined on a global time scale with a high degree of precision.
Owing to the bandwidth limitation of conventional transmitting and receiving systems, and to varying characteristics of the intermediate transmission paths, pulses received in an actual receiving system are—even if they were originally produced as ideal square-wave pulses—always subject to a certain distortion of their pulse form, hampering the establishment of a precise reception time. For range resolution in the millimetre range, for example, a time resolution in the picosecond range is required, which is not achievable with pulses distorted in this manner with a conventional threshold detection of the leading or trailing pulse edges. Refined methods have therefore already been proposed, such as using the focal point of a received pulse as the reception time, comparison of the pulse with a Gaussian pulse form (“Gauss fit”) or similar, which methods do indeed provide a higher time resolution, but one which can be very much improved upon.
The use of known methods for the measurement of distances by measurement of pulse transit time is rendered yet more difficult in that, depending on the distance and reflectivity (“black”, “white” or even “highly reflective”) of the target, the pulses reflected on a target (“echo pulses”) can—irrespective of whether optical, radio or acoustic pulses are involved—adopt an extremely large amplitude dynamic range, for example above 60 dB in the optical power and above 120 dB in the electrical output signal of a photodiode. Receiving systems that can process a dynamic range of such large proportions generally comprise compressed, i.e. strongly nonlinear, components in the receiving channel, and/or a plurality of parallel receiving channels with varying input sensitivities have to be provided in order to apportion the received signal to individual dynamic ranges that are easier to process further. Both methods lead to further distortion of the pulse form in the receiving system, making the precise establishment of the reception time yet more difficult.